Conversion of hydrocarbons



-long periods of time.

Patented Nov. 25, 1941 ooNvEnsIoN oF HYDRooARBoNs Crawford H. GreenewaltyWilmington, Del., assignor to E. `I.V du Pont de Nemours & Company,

. Wilmington, Del., a corporation of Delaware Application December 30, 1937, Serial No. 182,410

1 claim. (ci. 26o-eos) y This invention relates to the manufacture of low molecular weight oleiins and aromatics from petroleum, and more particularly, pertains to the production of normally gaseous oleiins such as ethylene and normally liquid aromatics such as benzene, toluene, and xylenes. 1

The fundamental principles applied by petroleum renners in producing gasoline and related products fromhydrocarbon oils by pyrolysis at elevated temperatures have been known for a long time. It is also well known that most processes which have previously been proposed formaking oleflns and aromatics by oil` cracking, convert relatively small proportions of the oil to olens or aromatcs and lyield the latter in a very impure state. It is well known that the effectiveness of many of these processes is based on cracking conditions which are so severe that sustained continuous operation is impossible because of failure of materials of construction or due to blocking of vital equipment parts with carbon. This invention constitutes an improvement over existolens, especially ethylene and/or propylene, from petroleum oil.' Another object is the simultaneous conversion of an appreciable portion of the oil to aromatic compounds and especially to benzene, toluene, and xylenes. `)A further object is the production of thesearomatic compounds ln`a relatively pure state and in relatively high yields, from `which undesirable .contaminants can be readilyremoved. A still further object is the achievement of the above ends under such conditions that continuous operation is possible for Other objects will appear hereinafter. 5

The above mentioned objects are accomplished by the following inventionwhich comprises the cracking of hydrocarbons suchA as an oil vapor, under conditions that produce lower oleflns and aromatic hydrocarbons, separating the products of cracking or these prcducts admixed with those of a separately operated gas. cracking process linto various fractions including a gaseous fracand to the accompanying drawing in which the preferred arrangement of apparatus, for carrying out the invention is illustrated diagrammatically. It is to be understood that details of this equipment which are familiar to those skilled in the art have beenomitted' for the sake of clarity,

that modification in constructural details and cept as may be required by the claim. i

Referring more particularly to the drawing, raw material which may, for example,`be gas-oil" or other petroleum oil, is delivered from a\suitab1e storage through line I to pump IA by means of which it is transferred via pipe line 2 tolwork tank 3; The fresh, raw materialis mixed 'in work tank 3 with other oil fractions which will be subsequently described. The `mixture leaves tank 3 through line I and is delivered by pump 5 through line 6 to preheating coil l. This and othercoils are suitably arranged inbanks in a. furnace setting. The preheating bank is preferably heated by the waste gases discharged from a combustion chamber of the Vfurnace in 'order that the gases vented to the stack will be ata minimum temperature. The charging stock is forced serially through the tubesof preheating bankl in order that the oil may be heated to a vvaporizing temperature ofapproximately 325 to 385 C. Ordinarily little thermal decomposition of the oil takes place in preheating coil l.

The material discharged from preheating bank 'l passes by way of pipe line 3 to an evaporator 9 wherein the high boiling constituents, which have remained in a liquid state at the preheater -exit temperature, are dropped to the base of the evaporator and withdrawn through outlet 9A and passechiirst'` through `coole`rI|l, and then by means of pipe line II to suitablestorage tanks. The lighter oils which are in a vaporized state in evaporator 9, pass overhead frein the latter through outlet line I2 and are introduceddnto ldrying bank I3 which is situated in an intermediate temperature zone in the furnace setting, and likepreheating coil 1, is largely heated by waste products of combustion.

The oil passes serially through the tubes which comprise the drying bank and its temperature is raised to about `4:60" to490f C. VThe temperature-time relationship in this bank is soadjusted that extensive cracking does not occur. Owing to inemcient separationof mist and vapor in the evaporator andto condensation in transfer line I2,` a small amount of liquid' oil is sometimes charged to the drying coils. This liquid,A however, is effectively evaporated in the drying bank, which discharges a substantially dryfvapor.

After being thoroughly `dried in coil I3, the hot vapors are introduced into a. bank of cracking T OFFICEv preferably about 690 to about 710 C. The vapors are maintained at this high temperature level in thelatter portion or soaking section of bank I4. It willfof course be realized that at this high reaction temperature,.precautions mustv be taken to avoid the deposition ol carbon in the reaction tubes and to prevent injuries to them. 'I'hese dii'iiculties are avoided by careful location of the tubes with respect to the furnace setting and burner llames, and by maintaining high velocities of oil vapor through the tubes in order that the heat applied to them may be promptly absorbed and carried away.

The hot reaction products, which have been held at a substantially constant temperature in the soaking section of I4, are discharged through pipe II into arrester It. The arrester is simply a chamber into which a cold, relatively volatile hydrocarbon fraction is introduced through a suitable spray nozzle. L The hot reaction products and the cold spray are intimately mixed, the latter is largely evaporated, and the temperature o! the reaction products is dropped almost instantaneously from about '100 to about 250 C. This quenching procedure terminates thecracking reaction abruptly. The mixture of quenched reaction products and vaporized quenching medium leaves arrester I6 by means of line l1 and is there Joined by a similar mixture which enters line I1 by way of line 5I. The material in line 5I consists of reaction products and quenching stockfrom a gas cracking operation which will be described in detail later. The above composite then passesyinto the base of tower I3. 'Ihe function of this tower is to separate the high boiling constituents formed during the cracking reaction, from unconverted charge and lower boiling materials. The heavy oils and tarry materials, which may find use asfuel oils, are discharged from the base of the tower i8 through outlet I! and cooler 20 and are nnally sent to storage vessels through pipe line 2|. The vapors which separate from the hot liquid at the base of tower I3 are scrubbed in the upper portion of the tower by descending, relatively volatile oil which has been introduced into the top of the column as reflux through line 29. The ter- 'minal' temperature conditions of tower. I8 are adjusted by varying the rates oi' quench liquid to arrester il-and of reiiux to the tower sothat thevyapors emerging from the top of the column to line 22 are free from heavy, tarry constituents and that the fuel oil fraction'discharge at the base is substantially free from volatile constituents which might have further value as cracking stock.

The vapors issuing from the top of tower I8 Y pass by means of'line, 22 to the base oi tower 23.

Its purpose is to separate-unconverted raw mate= rial or constituents in essentially the same boiling range from lower boiling liquids and iixed gas. 'I'he gas and vapors pass up through and are scrubbed by the liquid on the trays of the iractionating tower. .A light distillate having an end point of perhaps 150 to 200 C. is pumped into the top of tower 23 through line 38 to serve as reflux. Suiiicient reux is added in this manner so that the vapors, if cooled to condense constituents boiling in the gasoline range, give a condensate which has an end point of about 190 C. The higher boiling materials descendto the base of the fractionating tower where they are discharged by means of outlet line 24 through cooler 2l. A portion of the cooled bottoms from through line 21 and is introduced by way of line 26 into the top of tower I8 to serve as refiux there. The remainder returns by means of lines 26 and 2 to work tank 3 where it is mixed with fresh charge and the steps outlined above repeated to eiect further cracking.

The effluent vapors leave tower 23 through outlet 30, pass into condenser 3l, and are there cooled to about 30 C. The pressure at this point is about 10 lbs. per sq. in. gage. Under these conditions an appreciable amount of liquid is formed in the condenser and passes together with uncondensed vapors to a gas-liquid separator 33 by means of line 32. Light distillate is withdrawn from the baseY of this separator through outlet 34. A portion is delivered through line 35 to pump 36 which forces the material through line 31 and thence through lines 31A and 38 to furnish quenching medium and reux to the arrester I6 and fractionating tower 23, respectively. The light distillate made above that used for quenching and reuxing is withdrawn from the system to suitable storage tanks through line 34. This distillate is aromatic in character and may contain an aggregate of better than 50% benzene, toluene, and xylenes.

lA complex gas mixture containing hydrogen, methane, ethylene, ethane, propylene, propane, butylenes, and higher boiling hydrocarbons is withdrawn from the top of separator 33 through line 39, and passed into recovery'plant 40. Here the mixture is separated into the following fractions: (1) hydrogen and methane, (2) ethylene, (3) a mixture of ethane, C3 and C4 hydrocarbons, and (4) higher boiling lnfdrocarbons.`

The hydrogen and methane 4fraction Whichmay serve as fuel is'discharged from the re covery plant'through line 42. Ethylene is released via line 43 to suitable storage or directly to processes in which it may be used as raw matesure conditions is completely vaporized as it passes through the tubes of preheating bank 46 in which a final temperature of 650 to 700 C. is attained. The hot gases ar'e then introduced into a bank of cracking or conversion coils 41, which coils are maintained at a temperature of about 760 to about '790 C. These coils are preferably situated in a radiant section of the furnace setting. The time-temperature relationship in coils 41 is so regulated that about 50% of the incoming ethane, Cs and C4 hydrocarbons are converted to other compounds; for example, hydrogen, methane, ethane, ethylene, and hydrocarbons in the motor spirit boiling range. rlhe 50% conversion means the average of ethane, C: and C4 conversion, and not a 50% conversion of each constituent. High vapor velocities are employed in the conversion tubes to prevent excessive tube wall temperatures and to minimize carbon deposition.

The hot reaction products are discharged from the conversion zone 41 through line 48 into arrester 49. Arresters 49 and I6 may be similar the iractionating tower 23 passes to pump 28 75 in design. The products of gas cracking are remv detail later.

accanto duced instantly from reaction temperature to about 120 C. by quenching withavolatle fraction which is withdrawn from line 31 through line 50`tolthe arrester spray nozzle. The mixture of quenched reaction` products and quenching stock leaves arrester 49 by means of exit line `5I and/joins a similar mixture discharged from oil cracking arrester I6.

through the base ofthe ilash tower I8 via line l1. Fromthis point on, the equipment for processing the reaction products is common to bothV cracking stages and, with the exception of recovery plant l0, v,functions in the manner previously described. It should of course be realized that the inventionis not limited to the use of the same equipment for the separation of the products in ,each of the cracking steps.` Sepa.- rate equipment may, of course. be used.

The constituents comprisingthe higher boiling hydrocarbon fraction, which is separated inrecovery plant 40,'are now obtained from the gas fractions producedby oil and gas cracking operations. This" higher boiling mixture is discharged'from recovery `plant 40" through line 4l and may be mixed with the liquid irom'the gas and vliquid separator 33 `bv introducing sameI into j line 34 or the higherboiling fraction may be4 recovered as a separate fraction via line 52. By

this combination of oil cracking and gas cracking, it has been found thatthere is a markedincrease in yields of gaseous products from hydrogen to Cs hydrocarbons, inclusive, and of benzene, toluene, and xylenes over a process in which merely the oil cracking step is carried out.

The following example -is a specific illustration of the invention described herein:

Example 'A hydrocarbon distillate with characteristics similar to those of the petroleum fraction known to refiners as gas oil was used as a raw material in the process outlined above. This maing from the main combustion chamber of the furnace to its stack. The oil mixture was heated to 348`C.'in the preheating` section and was discharged at this temperature into theI lower part of an evaporator. `'I'his was a vertically mounted steel vessel with? a section above the 'oilinlet filled with Y packing rings. Eighty-five pounds per hour of steam were also Aadmitted to the evaporator. The highboiling'constituents of the hot oil mixture dropped to the base of the evaporator and were withdrawn and cooled at the rate oi 37.3 pounds per hour. Since the constituents of the recycle stock were relatively low boiling, the evaporator bottoms were mostly uncracked, high boiling ends from the fresh charge. The vaporized oil and steam passed overhead through the bed of packing, left the evaporator at 285 C., Vand then passed through the tubes of 'a drying coil vin which the temperature of the vapor was raised to 496" C. After leaving the drying coil, the vapors were trans.-

i The combined quenched products from oil cracking and gas cracking pass' mitted to a bank of cracking or conversion tubes which were arranged in series around the top wall of the main combustion chamber. Tlve temperature of the vapors wasraised rapidlyto 695 C. in the cracking zone and was maintained at substantially this temperatureln a soaking coil which was alsosituated in thev radiant sec,` tion of the furnace. l

The hotreaction products from the `soaking zone were discharged to an arrestorl chamber I and there intimately mixed with a cold spray of about 500 pounds per hour of a. light, distilled l hydrocarbon fraction. A This procedure gave a temperature of about 270"` C. t the arrestor exit, from which the mixture of quenched reaction products and vaporiz'ed quenching `stock was passed to the base of a fractionating tower. The quenched products from a second, or gascracking furnace, to be described later,` were" also received by the same tower. The heavy, tarry products of` oll and gas cracking accumulated at the bot-` tom of this tower, 4and'5441 pounds per hour;

suitable for `fuel oil, were withdrawn and cooled.. The lower boiling materials, including gas,-

steam, constituents in the gasoline boiling range,`

andunconverted and partially cracked charging stock. were taken oil overhead as vapor and passed into the base of a second fractionating column. Gas, light distillataand steam were dis tilled from the top ofthe second tower while higher boiling materials wereV discharged from its base and cooled. Five yhundred and thirty pounds per hour of thesecooled bottoms were pumped into the top ofthe ilrst fractionatingA tower' to serve as reflux there. The remainder was mixed with incoming'fresh feed oil in the proportions indicated earlienand recharged to the preheating zone oi the oil cracking furnace. The overhead vapor from the secondiractionator was cooled vin a condenser and the resulting ,mixture ofl vapor and condensate was discharged to a gas-liquid separating chamber.`

The two towers and the separator `were operated at a pressure of 8 to 10 pounds per sq. inch gage. Water, condensed from steam'admitted earlier in the process, was released from thebase of the separator. Light distillate was pumped from` this separator. to the top of -thesecond tower as reflux as welLas to` the arrestos oi' the oil cracking unit described above, `and also to :that of a gas cracking unit to be described below.

The excess of distillate was withdrawnffrom the` separator and passed to a storage tank.`

`Gaseous productsof oil and gas cracking,

amounting to3080 cu.ft./hr. (on water free ba-l sis at 0 C. and 1 atm. pressure) were vented from the top of the gas-liquid `separator `and conveyed to a. gas recovery plant. The crude gas had approximately the Vfollowing composition.

Mol. per cent.

H2 8.7 (II-I4 i 17.6 Calin 21.4 C2Hs-.. 16.4. v(Je 22.3 C4 A 7.3 C5 and higher 6.3

The gas was separated in the recovery plantV into the` following fractions: (1) hydrogen and methane, (2) ethylene. (3) a mixture ofethane,"

C3, and Cr hydrocarbons, and (4) a;` light aromatic ,distillate containing Cs and higher hydrocarbons. Fraction 3 was vaporized and forced together with 28 pounds per hour of steam into a bank of preheating tubes in a separate gas cracking furnace. The mixture of steam and hydrocarbon vapor was heated to 690 C. in the preheating z one and was then passed serially through the tubes of a cracking or conversion coil in which the vapors reached a temperature of 'Z50-'760 C. As in the case of they oil cracking furnace, the preheating coil was heated largely by convection and the conversion coil chiefly by radiation. Approximately 50% of the aggregate charge was converted to products other than CzHs, C3, and C4 hydrocarbons in a single passage through the gas cracking furnace. The temperature of the hot cracked -products was dropped almost instantaneously to 135 C. by quenching with cold distillate pumped from the gas-liquid separator into an arrestor at the exit of the gas cracking coil. The quenched products joined a similar mixture from the oil cracking operation and bothpwere fractionated into gas, light aromatic distillate, recycle stock, and refractory fuel oil in a common separation system. Fraction 4 above from the recovery plant, and the excess of condensate Withdrawn from the gas-liquid separator following the cracking units fractionating towers, amounted to 59.6 pounds per hour. This light distillate was very aromatic in character and contained much benzene, toluene, and xylene.

It has been previously mentioned that the bottoms obtained from the evaporator used in the oil cracking operation were substantially unvaporized fresh feed oil. The following yields were obtained based on 100 parts by weight of vaporized fresh feed.

Total *lrior to losses in gas recovery plant.

It should be understood that the successful application of this invention is not limited to the use of a single raw material. The use of-gas oil has been mentioned but other stocks such as crude petroleum and topped or reduced crude may also be employed. A viscosity breaking" operation may be used to advantage in conjunction with the oil cracking step if the raw material has a high pour point and high viscosity.

Steam may be used to adjust the contact time in the cracking sections of the oil and gas furnaces mentioned. The process, however, will function adequately without steam and its use is optional.

The invention pertains to improving the yield of oleiins and aromatic hydrocarbons being applicable to any cracking process where these products are obtained. In the cracking of hydrocarbons at temperatures in excess of 600 C. olefins and aromatic hydrocarbons are produced in substantial quantities, this invention is particularly applicable in such instances. The upper temperature limit of the cracking reactions in which oleflns and aromatic hydrocarbons are produced depends upon the length of time the hydrocarbons to be cracked remain under the cracking conditions. The shorter the time of cracking the higher the temperature that can be used, thus temperatures of about 900 C. have been used and it is possible to use even higher temperatures if the length of time of cracking is properly adjusted. The oil cracking may be carried out in the tubular type cracking units as disclosed herein at 600 to 750 C. but it is preferred to operate at S90-710 C. and to adjust the time-temperature relationship so that 40 to 60% of the total vaporized oil charge is cracked ina single passage through the cracking zone. It is also desirableto operate the gas cracking step in such a way that a per pass conversion of the same order of magnitude is achieved. In the tubular type cracking unit it is preferable to operate the gas cracking step at higher temperatures than the oil cracking step, usually at least C. higher. It is possible that the gas cracking step could be carried out at a temperature the same as or lower than the oil cracking step provided that the length of time of cracking in the gas cracking step is properly adjusted to produce olens and aromatic hydrocarbons.

It is desirable to maintain high vapor velocities in the high temperature zones to avoid over heating of cracking tubes and deposition of coke. It is also important from the standpoint of continuity of operation to effect an eilicient separation between cracked tar and lower boiling materials. The necessity of avoiding carbonaceous material in recycle stocks is familiar to those versed in the cracking art.

The production of ethylene has been emphasized but other hydrocarbons, such as the lower olens, can also be readily produced. For example any or all of the propylene in the raw gas may be separated in the gas recovery plant and withdrawn from the process. In this case, the C21-Is and C4 hydrocarbons, with or without the propylene originally present, would serve as charging stock for the separately operated gas cracking operation. The processis equally ilexible as regards the production of ethane or C4 hydrocarbons. The production of an aromatic distillate rich in benzene, toluene, and xylenes has been particularly mentioned but higher aromatics, including naphthalene, may be obtained by operating the fractionation equipment so that.

distillate of a higher end point is produced.

'.I'he invention is not limited to the particular point of separation which is made between the higher molecular weight fraction and the lower molecular Weight gaseous fraction which is cracked in the separate cracking step. The charging stock for the latter may contain a small amount of C5 constituents or may be relatively free from constituents higher in molecular Weight than C4.

The choice of the various conditions must depend on such factors as the nature of the primary raw material, the operating conditions maintained in the two cracking steps, and the relative quantities of the several products desired.

A very important advantage of this invention that high yields of gaseous olefins and of aromatics are obtained under such conditions that continuous operation is possible for long periods of time. Overheating and burning out of cracking tubes and excessive production of hydrogen and methane is avoided.

It is apparent that many widely diierent embodiments of this invention may be made without departing from the spirit and scope thereof and therefore it is noi; intended to be limited except as indicated in the appended claim.

I claim: y Theprocess for the production o! ethylene and aromatic hydrocarbons which comprises cracking a normally liquid hydrocarbon oil vapor deriv'ed trompetroleum oil` by passing said vapors through a cracking zone heated to a temperature y between 650 and 750 C., said passage being at such a rate that 40 to 60% of the vapors undergoes the cracking reaction. immediately reduc- `ing the temperature of` the hot gases from the `cracking zone to one not above 270 C. by quenching same with la cold spray of hydrocarbon distillate, separating the resulting cracked products into the following fractionsa fraction containing hydrogen and methanewa' second products;` cracking the fourth fractionL by passing same through a second cracking zone op`erv ated at-a temperature at least `50f' higher thanl the temperatureof the rst cracking zone ybut at a temperature not exceeding 800 C., said passage being at such a rate that 40 to 60% of the gas undergoes the cracking reaction, immediately fraction containing productolens, Aa". third iraction containing an aromatic `distillate, and a fourth fraction comprising essentially ethane l and C: and C4 hydrocarbons freed or the/desired reducing` the temperature of the gas from the cracking zone to a `temperature of about C. to about" C Mbyqunching same with a cold spray' oi.'v a hydrocarbon distillate in a quenching step separate from the: quenching step following 

